The present invention relates to improvements in antennas. In particular the present invention relates to broadband antenna of the Vivaldi, notch or tapered slot antenna family.
The Vivaldi antenna element was proposed by Gibson in 1979, (P. J. Gibson, The Vivaldi Aerial, in Proc. 9th European Microwave Conference, UK, June 1979, pp. 101–105). The original Vivaldi antennas were tapered notch antennas having notches which open in an exponential flare shape. They were constructed by conventional microwave lithographic thin film techniques on substrates having a high dielectric constant, for example, alumina. Gibson's work has subsequently developed to include high gain Vivaldi antennas constructed on ceramic substrates other than alumina which have high dielectric constants and on substrates having low dielectric constant, for example, plastics. Copper-clad plastics (cuclad), for example PTFE, RT/duroid (having a variety of values, typically r=2.2 or 2.94) or Kapton (r=3.5), are now conventionally used when ease of manufacture, surface adhesion and price are paramount. Alternatively conductive layers can be formed from other good conductors including gold and gold-plated copper.
The exponential flare shape was originally adopted to address a requirement for a constant beamwidth antenna which could cover the microwave frequency range between 2 GHz and 20 GHz. As Gibson explains in his paper, the shape taken by the edge of the tapered slot must be completely specified in terms of dimensionless normalised wavelength units for the beamwidth to be held constant. Exponential curves are good candidates for shapes specified in this way.
Approximations to constant beamwidth antennas can also be constructed using alternative types of curves in place of exponential curves; these alternatives include sinusoidal, parabolic, hyperbolic and polynomial curves. The edges of the slot can also be formed as straight lines in which case the antenna can also be called a longitudinal (or linear) tapered slot antenna (LTSA).
Any conventional tapered slot antenna is constructed from a thin conductive layer disposed by lithographic thin film techniques on a substrate. A slot, open at one end, (also known as a notch) is formed in the conductive layer and the gap between the sides of the slot widens from a minimum at the closed end of the slot, also known as a “stub”, to a maximum at the open end. In conventional Vivaldi antennas, the gap is mirror-symmetrical about an axis through the centre of the slot and each side of the conductive layer flares according to a predetermined exponential formula. The flared slot is an effective radiating element.
In operation, the antenna radiates preferentially from the open end of the notch in a direction away from the notch and along the axis of symmetry. The antenna may thus be classed as an endfire antenna.
Each region of conductive layer having a flare shaped edge will henceforth be referred to as a wing of the antenna due to the appearance of the conductive layer. It has been found effective to dispose two pairs of mirror-symmetrical wings on a thin substrate layer: one pair on either planar surface of the substrate layer. The pairs are preferably identical and the notch formed by one pair is preferably disposed parallel to the notch formed by the other pair.
The closed end of the slot line may be fed by any one of a variety of transmission lines including microstrip lines, striplines, fin-lines (as in waveguides) and probes. A microstrip transmission line generally comprises a track of conductor (usually copper) on an insulating substrate. On the reverse side of the substrate there is formed a ground plane (or “backplane”) of conductor which acts as the return conductor.
Certain arrangements of tapered slot antenna can be fed from two parallel strips of conductor on either surface of a flattened substrate in a transmission line formation know as a twinline feed. Variations on the Vivaldi antenna structure for which a twinline feed is appropriate include the (unbalanced) antipodal Vivaldi antenna and the balanced antipodal Vivaldi antenna.
In twinline fed antennas, the conductive wing regions are each arranged to have an inner edge and an outer edge. In the same way as the edge of the slot in a conventional Vivaldi antenna follows a flared curve, the inner edge of the conductive wing regions can be formed to conform to a similar flared curve. In contrast to the indefinite extent of the conductive layer away from the slot in a conventional Vivaldi antenna arrangement, a second outer edge can define the outer extent of each conductive wing. The outer edge too can be formed to follow a broader flared curve.
The (unbalanced) antipodal Vivaldi antenna was developed by Gazit in 1988 (E. Gazit, Improved design of the Vivaldi antenna, in IEE Proc., Vol. 135, Pt. H, No. 2, April 1988, pp 89–92) is constructed on a single sheet of microwave dielectric substrate and fed from a twinline. The conductor strip on one side of the twinline feeds a first wing on a first side of the substrate and the other conductor strip feeds a second wing on the second side of the substrate. The first and second wings are arranged so that, from a point of view at right angles to the plane of the substrate, there is a flare shaped slot.
The balanced antipodal Vivaldi antenna, developed by J. D. S. Langley, P. S. Hall and P. Newham in 1996, is constructed on a sandwich of at least two sheets of dielectric substrate and fed from a balanced twinline.
A balanced antipodal Vivaldi antenna can be constructed from a first wing on one side of a first sheet of dielectric substrate and a second wing on the other side of the first sheet. A second sheet of dielectric substrate is provided with a third wing on an outer side. The first sheet and second sheet are sandwiched together so that the first and third wings are outermost and so that a sheet of dielectric substrate is interposed between the first wing and the second wing and between the third wing and the second wing. The first and third wings are arranged to flare in a first curved shape. The second wing is arranged to flare in a second curved shape—the second curved shape being the mirror image of the first curved shape. When viewed at right angles to the plane of the substrates, the first and third wings on one side and the second wing on the other side form a flare shaped slot.
In theory, a Vivaldi antenna should radiate radio frequency electromagnetic waves at a given wavelength when the width of the widening slot (at right angles to the axis of symmetry) is approximately equal to half the wavelength. The performance of physical implementations of conventional antennas is degraded by a number of complicating factors. In particular, the edge of the flared slot becomes linear at either extreme of a limited range of frequencies.
It has been established experimentally that the conventional exponential flare shaped Vivaldi antenna has poor performance over ultra-wide bandwidths. The crisp radiation properties of the exponential flare break down both as operating frequency increases above the bounds of a characteristic range and as the frequency decreases below the bounds.
It has been noted that antennas constructed to the same basic exponential curve have a most reliable frequency range which depends upon the characteristic length scale of the antenna. To give concrete examples, an antenna having a maximum flare width of two centimeters has a relatively reliable performance over the frequency range 15–40 GHz while a larger antenna with a maximum flare width of the order of ten centimeters has a better performance at lower frequencies, between 1 and 10 GHz. In the example, the dielectric constant of the substrate used in both antennas was 2.94.
A perfect antenna would radiate electromagnetic waves of a given frequency at a point along the centre line of the slot for which the width of the widening slot is equal to half the wavelength corresponding to the given frequency. In the real world, antennas do not function so straightforwardly. As the given frequency increases, the point of radiation moves towards the closed end of the slot. As the slot narrows, the gradient of the exponential curve of the slot edge decreases in the direction of the closed end and becomes too shallow to radiate effectively.
On the other hand, as the given frequency decreases, the point of radiation moves towards the open end of the slot. As the slot becomes wider the gradient of the exponential curve increases and becomes too steep to radiate effectively.
It is therefore an object of the invention to obviate or at least mitigate the aforementioned problems.